In the winter and early spring of 1991, the Dutch pig industry was struck by a sudden outbreak of a new disease among breeding sows. Most sows showed anorexia, some aborted late in gestation (around day 110), showed stillbirths or gave birth to mummified fetuses and some had fever. Occasionally, sows with bluish ears were found, therefore, the disease was commonly named “Abortus Blauw”. The disease in the sows was often accompanied by respiratory distress and death of their young piglets and often by respiratory disease and growth retardation of older piglets and fattening pigs.
The cause of this epizootic was not known, but the symptoms resembled those of a similar disease occurring in Germany since late 1990, and resembled those of the so-called “Mystery Swine Disease” as seen since 1987 in the mid-west of the United States of America and in Canada (Hill, 1990). Various other names have been used for the disease; in Germany it is known as “Seuchenhafter Spätabort der Schweine” and in North America it is also known as “Mystery Pig Disease”, “Mysterious Reproductive Syndrome”, and “Swine Infertility and Respiratory Syndrome”. In North America, Loula (1990) described the general clinical signs as:                1) off feed, sick animals of all ages;        2) abortions, stillbirths, weak pigs, mummies;        3) post-farrowing respiratory problems; and        4) breeding problems.        
No causative agent has as yet been identified, but encephalomyocarditis virus (“EMCV”), porcine parvo virus (“PPV”), pseudorabies virus (“PRV”), swine influenza virus (“SIV”), bovine viral diarrhea virus (“BVDV”), hog cholera virus (“HCV”), porcine entero viruses (“PEV”), an influenza-like virus, chlamidiae, leptospirae, have all been named as a possible cause (Loula, 1990; Mengeling and Lager, 1990; among others).